Because flushes and straights are bigger than pairs, two-pair, and sets.
The goal is to win the most money possible, and having a multiway pot is a good way to win a lot of money, however I personally prefer to take drawing hands into multiway pots, in position. Regardless of which game I'm playing, if I think I'll get called around I'll raise. Or if the flop comes T-high with no 9, we're up against top pair x2 and a lesser overpair. If we call and spike an A the likelihood of us losing plummets to just barely, and on a flop of say, AKT we're really only worried about a Jack to come.
We don't know what kind of game we're playing (tournament or cash), what stack sizes are, and what player frequencies are. Well, that depends, we only know what our villains have, not anything about them. T9s opens with a raise from MP, QT calls from CO, KK's on the button and 3-bets, and we have AA. Let's say we're in that hand he posted, with the QT, the T9, and the KK. I've got the entire Roy Rounders 'buy my stuff' e-mail list in my inbox, I still glaze over it time to time, but most of his stuff is very situation specific.
